Motion compensated spiral FISP MRI

ABSTRACT

Real-time imaging of a moving object such as the heart uses fast imaging with steady precession (FISP) traversing spirals in k-space. After flipping nuclear spins in the object within a slice to be imaged, signals are read out from the nuclear spins while applying read-out magnetic field gradients whereby read-out signals traverse spirals in k-space. Thereafter, the zero moment and first moment of the read-out gradients are driven to zero quickly so that fast imaging with steady state precession is realized without banding artifacts. Motion compensated rewinders are applied after the read-out magnetic field gradients which can be integral with the read-out gradients or comprise separate compensation lobes.

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to magnetic resonance imaging(MRI), and more particularly the invention relates to the use of fastimaging with steady precession (FISP) and with motion compensated spiralk-space trajectories.

[0002] In cardiac imaging using MRI techniques, rapid acquisition of MRIsignals is desired. MR ventriculography is often performed in a gatedmode using conventional gradient echo 2DFT techniques. These techniquesoften suffer from inadequate signal and inadequate contrast betweenblood and myocardium. Recently, MR scanners have been including amodification of these techniques using steady-state free precession(SSFP) or “FISP” techniques, which refocus all gradient lobes betweenexcitations to preserve transverse magnetization for a longer time.These techniques lead to increased signal and increased blood-myocardiumcontrast, but cardiac-gating and breath-holding are still required toget good spatial and temporal resolution, because of the inherentinefficiencies of 2DFT scanning.

[0003] Fluoroscopic ventriculography is advantageous because cardiacgating and breath-holding are not needed and because it can be used in areal-time interactive mode, similar to echocardiography. Fluoroscopicventriculography requires an efficient scanning technique, such asspiral scanning. However, conventional spiral ventriculography sometimessuffers from inadequate contrast between blood and myocardium andinadequate signal.

[0004] It would be advantageous to use spiral k-space scanningtechniques to improve image contrast. However, current spiral scanningtechniques use a repetition time (TR) of 25-40 ms, because of longspectral-spatial excitation pulses used for fat suppression and becauselonger readouts allow for increased scan efficiency. On the other hand,FISP techniques require short repetition times, on the order of 2-6 ms,since with longer repetition times image banding occurs due to fieldinhomogeneity. Thus, spiral ventriculography and FISP techniques wouldappear to be incompatible.

[0005] The present invention is directed to the practical use of FISPtechniques and spiral k-space scanning in motion compensated cardiacimaging.

BRIEF SUMMARY OF THE INVENTION

[0006] In accordance with the invention, spiral scanning with reducedrepetition time is combined with FISP MRI acquisition sequences. Theshorter TR prevents FISP banding artifacts in the images.

[0007] A feature of the invention is the design of spiral magnetic fieldgradients whereby k-space is scanned in a very short readout period. Thegradients are then returned to zero amplitude in a manner to eliminategradient moments. The zero moment must be zeroed out for allapplications in order to maintain SSFP or FISP coherence. Forventriculography applications, the first moment must also be zeroed outto avoid image artifacts from moving structures or flowing blood.

[0008] To zero out both the zero and first moments in a conventionalmanner would require a number of additional gradient lobes, which couldpush the repetition time, TR, up so that FISP image banding artifactsappear. It would also increase higher order moments significantly. Inaccordance with one feature of the invention an optimized k-spacerewinder is provided that simultaneously nulls the zeroth and firstmoments while returning the gradient amplitude to zero.

[0009] The invention and objects and features thereof will be morereadily apparent from the following detailed description and appendedclaims when taken with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 illustrates an excitation pulse and magnetic fieldgradients for real-time spiral SSFP (FISP) cardiac imaging in accordancewith an embodiment of the invention.

[0011]FIGS. 2A and 2B illustrate a conventional magnetic field gradientspiral rewinder and a rewinder with a separate bipolar for motioncompensation.

[0012]FIG. 3 illustrates a magnetic field gradient with built-in motioncompensation in accordance with an embodiment of the invention.

[0013]FIGS. 4A, 4B illustrate the design of an x-gradient rewinder and ay-gradient rewinder in accordance with an embodiment of the invention.

[0014]FIGS. 5A, 5B illustrate spiral SSFP images without and with amotion compensated rewinder.

[0015]FIGS. 6A, 6B are spiral SSFP images without and with motioncompensated rewinders for a patient with mild coronary artery disease.

[0016]FIGS. 7A, 7B illustrate spiral non-SSFP and spiral SSFP images ofa patient with left main and right coronary artery disease.

[0017]FIGS. 8A, 8B are spiral non-SSFP and spiral SSFP images of apatient with left main and right coronary artery disease.

[0018]FIGS. 9A, 9B are spiral non-SSFP and spiral SSFP images of apatient with left main and right coronary artery disease.

[0019]FIGS. 10A, 10B are spiral non-SSFP and spiral SSFP images of aheart transplant patient.

[0020]FIGS. 11A, 11B are spiral non-SSFP and spiral SSFP of the hearttransplant patient.

DETAILED DESCRIPTION OF THE INVENTION

[0021] Referring now to the drawings, FIG. 1 illustrates a pulsesequence in accordance with the invention for combining the contrast ofSSFP (FISP) with the efficiency of spiral k-space scanning. An RF pulseis applied in combination with G_(z) gradient pulses for tilting nuclearspins in an object of interest with slide selection determined by theG_(z) gradient. Signals from the precession of the tilted nuclear spinsare read out in the presence of G_(x) and G_(y) gradients. Each of theG_(x) and G_(y) gradients includes at the end thereof a rewinder withbuilt-in motion compensation which optimizes for a minimum repetitiontime, TR, for decreased scan time and decreased sensitivity toinhomogeneity. Further, higher order moments are reduced to therebyreduce sensitivity to acceleration.

[0022] Conventionally, spiral rewinders are provided with G_(x) andG_(y) gradients to bring the k-space scan back to the k-space origin asshown in FIG. 2A. Further, a spiral rewinder plus separate bipolarportions have been provided for motion compensation as shown in FIG. 2B.However, in accordance with the invention the rewinder for the G_(x) andG_(y) gradients as shown in FIG. 3 has built-in motion compensationwhich is optimized for a minimum repetition time and decreased scan timeand provides decreased sensitivity to inhomogeneity. Further, the zeroand first order moments are nulled to zero while higher order momentsare reduced for a reduced sensitivity to acceleration.

[0023] For the motion compensated rewinder design, each of the x and yrewinders are separately optimized for a constant slew rate, S. The timeorigin for the moment calculations is at the end of readout which is thestart of the rewinder.

[0024] The values of the zero moment and first moment of the readoutgradient waveforms are given as follows:M₀ = ∫_(−T)⁰G(t)  t  M₁ = ∫_(−T)⁰t  G(t)  t  

[0025] In order to null these moments, the rewinder gradient waveformsmust have moments that are the inverse of those of the preceding readoutgradient waveforms, with the same time reference at the start of therewinder.

[0026] Before starting the design of the rewinders, for ease ofcalculation we first rotate the spiral readout gradients to assume thegeneral form shown in FIG. 4, wherein the Gy gradient has approximatelyzero magnitude. At this point, typically the Gx gradient is near itsmaximum magnitude. If desired, the gradients can be rotated back totheir original position after the rewinder lobes are added. In designingthe x-rewinder shown in FIG. 4, the values of τ₁, τ₂, and τ₃ in thefollowing equations are solved: $\begin{matrix}{\tau_{1} = \frac{G_{1}}{S}} \\{{- \frac{M_{0}}{S}} = {{\frac{1}{2}\tau_{1}^{2}} - \tau_{2}^{2} + \tau_{3}^{2}}} \\{{- \frac{M_{1}}{S}} = {{\frac{1}{6}\tau_{1}^{3}} - {\tau_{2}^{2}\left( {\tau_{1} + \tau_{2}} \right)} + {\tau_{3}^{2}\left( {\tau_{1} + {2\tau_{2}} + \tau_{3}} \right)}}}\end{matrix}$

[0027] The y-rewinder as shown in FIG. 4B has unknowns τ₁ and τ₂ whichare solved as follows: $\begin{matrix}{{- \frac{M_{0}}{S}} = {\tau_{1}^{2} - \tau_{2}^{2}}} \\{{- \frac{M_{1}}{S}} = {\tau_{1}^{3} - {2\tau_{1}\tau_{2}^{2}} - \tau_{2}^{3}}}\end{matrix}$

[0028] The x and y rewinders as calculated above provide zero first andsecond moments to maintain coherence and to avoid build-up of motionsensitivity over multiple repetition times.

[0029] FIGS. 5-11 are images of several patients under different imagingconditions. The images were obtained using a 1.5T GE Signa CV/I scannerusing 40 mT/m gradients with 150 T/m/s slew rate. A repetition time of4.5 ms with a flip angle of 60° and slice width of 5-10 mm was employedwith 20 spiral k-space interleaves. Eleven true frames per second wereobtained with a field of view of 24 cm and 2.35 mm spatial resolution.

[0030]FIGS. 5A and 5B are images of a volunteer along the long axesusing spiral SSFP without a motion compensated rewinder and with amotion compensated rewinder in accordance with the invention. FIGS. 6A,6B are images of a patient with mild coronary artery disease with spiralSSFP without a motion compensated rewinder and spiral SSFP with motioncompensated rewinder. Improved image quality is noted in FIG. 6B usingspiral SSFP with a motion compensated rewinder in accordance with theinvention.

[0031] Comparison of the invention with non-SSFP spiral sequences wasalso obtained using 21 mT/m maximum gradient with 120 T/m/s slew rate.The repetition time, TR, was 30 ms with a flip angle of 30° and slicewidth of 7 mm. Six spiral interleaves were obtained with 5.6 trueframes/s and 16 frames/s with a sliding window. The field of view was 24cm with 2.24 mm spatial resolution. FIGS. 7A, 7B illustrate a patientwith left main and right coronary artery disease using spiral non-SSFPimaging and spiral SSFP in accordance with the invention, respectively.FIGS. 8A, 8B are also images of the patient with right coronary arterydisease using spiral non-SSFP imaging and spiral SSFP imaging inaccordance with the invention, respectively. Similarly, FIGS. 9A, 9B areimages using spiral non-SSFP imaging and spiral SSFP imaging of thepatient with right coronary artery disease.

[0032]FIGS. 10A, 10B are images of a heart transplant patient usingspiral non-SSFP imaging and spiral SSFP imaging in accordance with theinvention, and FIGS. 11A, 11B are other images of the heart transplantpatient using spiral non-SSFP and spiral SSFP, respectively.

[0033] By employing gradient rewinders with built-in motion compensationin accordance with the invention, real-time cardiac imaging sequencesutilize the contrast of SSFP, the speed of FISP, along with theefficiency of k-space spiral scanning. Image banding is avoided bykeeping the repetition time short through optimized gradient waveformdesign. SSFP coherence is maintained by driving the zeroth gradientmoments to zero. Motion and flow artifacts are reduced by driving thefirst gradient moments to zero and reducing higher order moments. Whilethe invention has been described with reference to specific embodiments,the description is illustrative of the invention and is not to beconstrued as limiting the invention. Various modifications andapplications may occur to those skilled in the art without departingfrom the true spirit and scope of the invention as defined by theappended claims.

What is claimed is:
 1. A method of real-time imaging in an object usingmagnetic resonance comprising the steps of: a) flipping nuclear spins inthe object within a slice to be imaged, b) reading out signals from thenuclear spins while applying read-out magnetic field gradients wherebyread-out signals traverse spirals in k-space, and c) driving the zeromoment and first moment of the read-out gradients to zero after readingout signals whereby fast imaging with steady precession (FISP) isrealized without banding artifacts.
 2. The method as defined by claim 1wherein step c) includes applying motion compensated rewinders after theread-out magnetic field gradients.
 3. The method as defined by claim 2wherein one rewinder is designed for a constant slew rate, S, a zeromoment, −M₀, a first moment, −M₁, and with a time origin at the end ofread-out and zero value spacing of τ₁, 2τ₂, and 2τ₃ where$\begin{matrix}{\tau_{1} = \frac{G_{1}}{S}} \\{{- \frac{M_{0}}{S}} = {{\frac{1}{2}\tau_{1}^{2}} - \tau_{2}^{2} + \tau_{3}^{2}}} \\{{- \frac{M_{1}}{S}} = {{\frac{1}{6}\tau_{1}^{3}} - {\tau_{2}^{2}\left( {\tau_{1} + \tau_{2}} \right)} + {\tau_{3}^{2}\left( {\tau_{1} + {2\tau_{2}} + \tau_{3}} \right)}}}\end{matrix}$


4. The method as defined by claim 3 wherein another rewinder is designedfor a constant slew rate, S, with a time origin at the end of read-outand zero value spacing of 2τ₁ and 2τ₂ where $\begin{matrix}{{- \frac{M_{0}}{S}} = {\tau_{1}^{2} - \tau_{2}^{2}}} \\{{- \frac{M_{1}}{S}} = {\tau_{1}^{3} - {2\tau_{1}\tau_{2}^{2}} - \tau_{2}^{3}}}\end{matrix}$


5. The method as defined by claim 4 wherein flip angle is 60°,repetition time is 4.5 ms, imaged slice is 5-10 mm, and read-outgradients are 40 mT/m with 150 T/m/s slew rate.
 6. The method as definedby claim 4 wherein the zero moment, M₀, is: M₀ = ∫_(−T)⁰G(t)  t  

and the first moment, M₁, is:   M₁ = ∫_(−T)⁰t  G(t)  t  .


7. The method as defined by claim 6 wherein the motion compensatedrewinders are integral with the read-out gradients.
 8. The method asdefined by claim 6 wherein the motion compensated rewinders compriseseparate compensation lobes.
 9. The method as defined by claim 2 whereinthe motion compensated rewinders are integral with the read-outgradients.
 10. The method as defined by claim 2 wherein the motioncompensated rewinders comprise separate compensation lobes.
 11. Themethod as defined by claim 1 wherein the imaging is cardiac imaging. 12.Apparatus for real-time imaging in an object using magnetic resonancecomprising the steps of: a) means for flipping nuclear spins in theobject within a slice to be imaged, b) means for reading out signalsfrom the nuclear spins while applying read-out magnetic field gradientswhereby read-out signals traverse spirals in k-space, and c) means fordriving the zero moment and first moment of the read-out gradients tozero after reading out signals whereby fast imaging with steadyprecession (FISP) is realized without banding artifacts.
 13. Apparatusas defined by claim 12 wherein element c) includes applying motioncompensated rewinders after the read-out magnetic field gradients. 14.Apparatus as defined by claim 13 wherein one rewinder is designed for aconstant slew rate, S, a zero moment, −M₀, a first moment, −M₁, and witha time origin at the end of read-out and zero value spacing of τ₁, 2τ₂,and 2τ₃ where $\begin{matrix}{\tau_{1} = \frac{G_{1}}{S}} \\{{- \frac{M_{0}}{S}} = {{\frac{1}{2}\tau_{1}^{2}} - \tau_{2}^{2} + \tau_{3}^{2}}} \\{{- \frac{M_{1}}{S}} = {{\frac{1}{6}\tau_{1}^{3}} - {\tau_{2}^{2}\left( {\tau_{1} + \tau_{2}} \right)} + {\tau_{3}^{2}\left( {\tau_{1} + {2\tau_{2}} + \tau_{3}} \right)}}}\end{matrix}$


15. Apparatus as defined by claim 14 wherein another rewinder isdesigned for a constant slew rate, S, with a time origin at the end ofread-out and zero value spacing of 2τ₁ and 2τ₂ where $\begin{matrix}{{- \frac{M_{0}}{S}} = {\tau_{1}^{2} - \tau_{2}^{2}}} \\{{- \frac{M_{1}}{S}} = {\tau_{1}^{3} - {2\tau_{1}\tau_{2}^{2}} - \tau_{2}^{3}}}\end{matrix}$


16. Apparatus as defined by claim 15 wherein flip angle is 60°,repetition time is 4.5 ms, imaged slice is 5-10 mm, and read-outgradients are 40 mT/m with 150 T/m/s slew rate.
 17. Apparatus as definedby claim 15 wherein the zero moment, M₀, is: M₀ = ∫_(−T)⁰G(t)  t  

and the first moment, M₁, is: M₁ = ∫_(−T)⁰tG(t)  t.


18. Apparatus as defined by claim 17 wherein the motion compensatedrewinders are integral with the read-out gradients.
 19. Apparatus asdefined by claim 17 wherein the motion compensated rewinders compriseseparate compensation lobes.
 20. Apparatus as defined by claim 13wherein the motion compensated rewinders are integral with the read-outgradients.
 21. Apparatus as defined by claim 13 wherein the motioncompensated rewinders comprise separate compensation lobes. 22.Apparatus as defined by claim 12 wherein the imaging is cardiac imaging.